Aggregates, Crystals, Gels, and Amyloids: Intracellular and Extracellular Phenotypes at the Crossroads of Immunoglobulin Physicochemical Property and Cell Physiology

Recombinant immunoglobulins comprise an important class of human therapeutics. Although specific immunoglobulins can be purposefully raised against desired antigen targets by various methods, identifying an immunoglobulin clone that simultaneously possesses potent therapeutic activities and desirable manufacturing-related attributes often turns out to be challenging. The variable domains of individual immunoglobulins primarily define the unique antigen specificities and binding affinities inherent to each clone. The primary sequence of the variable domains also specifies the unique physicochemical properties that modulate various aspects of individual immunoglobulin life cycle, starting from the biosynthetic steps in the endoplasmic reticulum, secretory pathway trafficking, secretion, and the fate in the extracellular space and in the endosome-lysosome system. Because of the diverse repertoire of immunoglobulin physicochemical properties, some immunoglobulin clones’ intrinsic properties may manifest as intriguing cellular phenotypes, unusual solution behaviors, and serious pathologic outcomes that are of scientific and clinical importance. To gain renewed insights into identifying manufacturable therapeutic antibodies, this paper catalogs important intracellular and extracellular phenotypes induced by various subsets of immunoglobulin clones occupying different niches of diverse physicochemical repertoire space. Both intrinsic and extrinsic factors that make certain immunoglobulin clones desirable or undesirable for large-scale manufacturing and therapeutic use are summarized. 1. Introduction Immunoglobulins (Igs) are the important mediators of humoral immunity. Because of their variable domain primary sequence diversity that is somatically generated via combinatorial gene segment joining of germline-encoded DNA and hypermutations [1], the repertoire of Ig clones is estimated to exceed 1010 [2]. Such sequence diversity is advantageous when it comes to conferring comprehensive foreign antigen coverage to protect the host, but the same sequence variations—in conjunction with a structural constraint of a scaffold-based protein design—can pose a significant biosynthetic challenge to produce and secrete each Ig clone in an equally efficient manner. Because of the sequence diversity within the variable domains, individual Igs possess different physicochemical attributes unique to each clone that may impose differential resource load to the cell and thus may modulate various biosynthetic processes, including the rate of protein synthesis, folding kinetics,